1. Field of the Invention
The present invention relates to a stent for expanding a lumen of a body.
2. Description of the Prior Art
It is desirable in various situations that means has to be provided for expanding a constricted vessel portion or for maintaining an open passageway through a vessel portion.
For example, these situations can be those by malignant esophageal strictures that are caused by esophageal carcinoma or esophageal metastasis, those by benign strictures that are caused by operation or cauterant esophagitis, or those by strictures of blood vessel system, biliary system, lachrymal duct system, urinary duct system and bronchial system.
The balloon expansion has been a well-known method of enlarging and maintaining the strictured site in these cases. However, such method has a disadvantage in that it is used repeatedly on a patient due to its temporary effects, and it has no effects on the patients of serious strictures.
As alternatives, various artificial-esophagi have been used in the cases of esophageal strictures, but since they have no constriction and relaxation, the rate of esophageal rupture is high (30-40%) in the process of inserting them into the strictured site that has been caused by cancer, and the high mortality rate due to mediastinitis results from rupture of an esophagus. In addition, the patient has a great difficulty in swallowing due to a narrow inner diameter (10-12 mm) of artificial esophagus, and obstruction of an artificial esophagus occurred frequently due to food intakes.
As the means of overcoming the difficulty, a device to hold the passageway enlarged using a stent was presented by U.S. Pat. No. 4,214,587. However, so the device of the invention has the temporary effect in enlarging the passageway, there is still the problem that the endovascular lumen gets narrows after a long time.
To improve this disadvantages, U.S. Pat. No. 4,580,568 discloses a stent including a wire formed into a closed zigzag configuration including an endless series of straight sections joined by bends. The stent is resiliently compressible into a smaller first shape wherein the straight sections are arranged side-by-side and closely adjacent one another for insertion, and into a larger second shape wherein the straight sections press against the walls of the passageway to maintain it open.
Self-expandable stents are normally evaluated with respect to four performance characteristics: the radially outward expandable force that the stent exerts on the vascular wall; the small diameter to which the stent is capable of being compressed for the insertion procedure; the ability of the stent to adapt to curved passageways in the patient's body; and the stability of the stent in not migrating from its originally implanted position within the patient.
Conventional zigzag stents must normally be made relatively short because the straight wire sections prevent the stent from readily adapting to curves in the passageway of a patient. Furthermore, the expandable force of conventional zigzag stents generally decreases with the length of the stent. One solution to these drawbacks has been to modify the conventional zigzag stent by connecting a plurality of shorter stents end on end to create one longer zigzag stent assembly. Although these modified zigzag stents for certain applications, there exists a need for an elongated self-expandable stent that includes the advantages of both conventional and modified zigzag stents but which has improved performance characteristics over both.
To solve these problems, a number of stents have been developed, wherein there are stents of Wall-type and Cook-type as a typical one among them.
International Publication No. WO 93/13825 discloses an example of the Cook type of stent. The self-expandable stent disclosed in the Publication includes a wire bent into an elongated zigzag pattern having a plurality of substantially straight wire sections separating a plurality of bends, and a plurality of filaments for interconnecting adjacent bends of the helix. The elongated zigzag pattern is helically wound about a center axis to define a tubular shape such that a majority of the plurality of bends is disposed in a helix. There is a drawback in that the stent has a good expanding force but poor structural stability, because the plurality of filaments is interconnected each other into a zigzag pattern.
U.S. Pat. No. 4,655,771 discloses an example of the Wall type of stent. The stent disclosed in the patent has a radial and axially flexible, elastic tubular body with a predetermined diameter that is variable under axial movement of ends of the body relative to each other and which is composed of a plurality of individually rigid but flexible and elastic thread elements each of which extends in a helix configuration along the center line of the body as a common axis, the flexible and elastic elements defining radially self-expanding body. The body includes a first number of elements having a common direction of winding but be axially displaced relative to each other, and crossing a second number of elements also axially displaced relative to each other but having an opposite direction of winding, the crossing of the first and second elements defining at least an obtuse angle. The stent has a uniform and stable structure, but has a disadvantage that the expandable force deteriorates at some extent. In addition, the zigzag pattern stent generally shortens axially as it radially expands.